To respond to an increase in data traffic typically found in the Internet, construction of high-speed and large-capacity optical access networks rapidly progresses. Recently, a gigabit-passive optical network (G-PON) in which a plurality of subscribers share an optical fiber cable while high-speed upstream transmission of 1.25 Gbps and downstream transmission of 2.4 Gbps at the maximum is available is widely used as a high-speed optical access system for constructing an optical access network.
To achieve still higher speed transmission in the future, development of a bit-rate mixed PON system in which an optical access network that has already been constructed is utilized and at the same time high-speed transmission (for example, 10 Gbps) is available only to the subscribers who require high-speed transmission is expected from the economical point of view. For example, Japanese Laid-open Patent Publication No. 08-008954 discloses a technology that implements a bit-rate mixed PON system. According to the technology, a multi-rate burst circuit is provided to an optical network unit (ONU) for each subscriber. Thus, a plurality of bit rates can be mixed in the PON system.
If an existing optical access network is utilized so that the optical access network contains a plurality of bit rates, a subscriber that uses a high-speed bit rate encounters receiver sensitivity degradation. The faster a network is, i.e., the broader a band is, the lower a signal/noise (S/N) ratio becomes. Therefore, by speeding up part of an existing optical access network that has been designed such that the network has a relatively low receiver sensitivity without expecting higher speed data transmission, the S/N ratio drops due to the widening of the band, which degrades the receiver sensitivity.
For example, if part of an optical access network that has been designed to run at 2.4 Gbps is speeded up to 10 Gbps, receiver sensitivity of a communication terminal apparatus for a subscriber using the speed-up transmission drops, for example, by about 4 dB. As a result, expected high transmission speed may not be achieved.
Similarly, in a network formed from an existing optical access network so that higher speed communication is available for some of the subscribers of the network, receiver sensitivity degradation occurs also in an optical line terminal (OLT) that functions as a line concentrator located on the station side. In other words, if the OLT is speeded up (broadbanded) to receive an upstream signal from a subscriber that uses higher speed communication, the S/N ratio deteriorates as the band is broadened. As a result, reception of an upstream signal from an existing ONU designed in relatively low receiver sensitivity becomes more difficult.